1. Field of the Invention
The present invention relates generally to methods for forming a thin film, a gate electrode, and a transistor, and a gate electrode formed by these methods, and more particularly to a method for forming a thin film that includes nitride on a gate electrode pattern that has a polycide structure, a method for forming a gate electrode that can reduce oxidation of the gate electrode during a re-oxidation process, a method of forming a transistor that has a lightly doped drain structure, and a gate electrode that does not contain any defects after a re-oxidation process.
2. Description of the Related Art
Semiconductor devices have been developed to have high integration densities. As a result, the sizes of the patterns formed on a semiconductor substrate and the intervals between the patterns have been greatly reduced. In the past, polysilicon was widely employed as the wiring material for forming a gate electrode or a bit line. However, because polysilicon has a high specific resistance, a gate electrode formed of polysilicon has disadvantages such as an RC time delay and an IR voltage drop in accordance with the reduction in the sizes of the patterns on the substrate. A polycide having characteristics similar to those of a polysilicon has been used as the wiring material in a gate electrode or a bit line in very large scale integration (VLSI) circuits because the polycide has a specific resistance several times lower that that of polysilicon. The polycide has the composite structure of polysilicon and a refractory metal silicide.
The metal silicide includes a refractory metal, such as, for example, tungsten (W), molybdenum (Mo), titanium (Ti), or tantalum (Ta). Such a metal silicide is used as a low resistance wiring material in the formation of the VLSI circuit. The metal silicide is coupled to a polysilicon doped with a high concentration to be used for forming a gate electrode.
When a polycide structure is employed for the gate electrode, it is difficult to perform a re-oxidation process to fix the damage to a gate oxide film and a semiconductor substrate after the formation of the gate electrode because the refractory metal in the metal silicide is oxidized during the re-oxidation process. Examples of gate electrodes having polycide structures are disclosed in U.S. Pat. No. 5,706,164 (issued to Jeng) and U.S. Pat. No. 5,710,454 (issued to Wu).
FIG. 1 is cross-sectional view illustrating a gate electrode formed according to a conventional method. After forming a gate oxide film 102 on a semiconductor substrate 100, a polysilicon layer, a metal silicide layer, and a silicon nitride layer are subsequently formed on the gate oxide film 102. The polysilicon layer, the metal silicide layer and the silicon nitride layer are then patterned via a photolithography process to form a gate electrode 10 having a polysilicon pattern 104, a metal silicide pattern 106, and a silicon nitride pattern 108.
When performing a re-oxidation process, the metal silicide is oxidized and defects such as abnormal oxidation or the metal silicide deforming into the shape of an ear (A) may occur. The ear-shaped defect of the metal silicide (A) may deteriorate the operation of the gate electrode 10 and may reduce the margin for filling the gap between the gate electrodes 10. Therefore, a void between the gate electrodes 10 may occur when the space between the gate electrodes is filled with an insulation material such as silicon oxide.
To overcome the above-mentioned problems, the gate electrode is thermally treated at a temperature of at least 900° C. When the gate electrode is treated at such a high temperature, abnormal oxidation of the gate electrode can be somewhat reduced. However, this high temperature treatment for the gate electrode may introduce a fatal failure in the operation of a transistor having a short channel structure or a lightly doped drain (LDD) structure. Thus, it is difficult to form a gate electrode having a polycide structure without defects.